Enhanced photo-fenton and photoelectrochemical activities in nitrogen doped brownmillerite KBiFe2O5

Visible-light-driven photo-fenton-like catalytic activity and photoelectrochemical (PEC) performance of nitrogen-doped brownmillerite KBiFe2O5 (KBFO) are investigated. The effective optical bandgap of KBFO reduces from 1.67 to 1.60 eV post N-doping, enabling both enhancement of visible light absorption and photoactivity. The photo-fenton activity of KBFO and N-doped KBFO samples were analysed by degrading effluents like Methylene Blue (MB), Bisphenol-A (BPA) and antibiotics such as Norfloxacin (NOX) and Doxycycline (DOX). 20 mmol of Nitrogen-doped KBFO (20N-KBFO) exhibits enhanced catalytic activity while degrading MB. 20N-KBFO sample is further tested for degradation of Bisphenol-A and antibiotics in the presence of H2O2 and chelating agent L-cysteine. Under optimum conditions, MB, BPA, and NOX, and DOX are degraded by 99.5% (0.042 min-1), 83% (0.016 min-1), 72% (0.011 min-1) and 95% (0.026 min-1) of its initial concentration respectively. Photocurrent density of 20N-KBFO improves to 8.83 mA/cm2 from 4.31 mA/cm2 for pure KBFO. Photocatalytic and photoelectrochemical (PEC) properties of N-doped KBFO make it a promising candidate for energy and environmental applications.

Contaminants like organic dyes, synthetic compounds and antibiotics in wastewater are severe threat to environment and human health [1][2][3] . Several organic dyes have been used as a human and veterinary medicine for some of therapeutic and diagnostic procedures 4,5 . However, traces of dyes in water bodies is hazardous to environment and difficult to degrade using conventional water treatment methods due to aromatic structures, hydrophilic nature and high stability against light, and temperature etc 6 . Another organic effluent Bisphenol-A [2,2-bis (4-hydroxyphenyl) propane] or BPA widely found in wastewaters, is a raw material for manufacturing epoxy and polycarbonate plastics. Recent studies reveal that BPA has severe effects on the human health. , effects reproductive systems and causes fertility problems 7,8 . It is one of the emerging pollutants, contaminating water bodies in recent times due to excessive plastic usage. This synthetic compound is difficult to degrade in natural conditions due to its complex structure. Various techniques such as physical adsorption, biodegradation and other chemical remediation are tested for degradation of BPA, which are expensive as well as take longer time to degrade 9 . Hence, economical and energy efficient strategies are required to treat these kinds of effluents. Water pollutants like pharmaceutically active compounds such as antibiotics are also being extensively used in recent times for the treatment of infectious diseases and for enhancing agricultural production 10,11 . Their extensive use, incomplete biodegradability, partial removal using conventional water treatment plants lead to environmental contamination. Some of such antibiotics are Norfloxacin (NOX) and Doxycycline (DOX). Norfloxacin is a Fluoroquinolone antibiotic widely used for respiratory and bacterial infections 12 . Doxycycline is one of the widely used antibiotic, which is used to treat some of the most hazardous diseases such as plague and anthrax 13 . These fluoroquinolone and Doxycycline antibiotics are widely detected in surface water and other environmental matrixes due to incomplete treatment of these antibiotics in water treatment plants. A prolonged exposure to these antibiotics in aquatic environment can lead to antibiotic resistance 14,15 . As a result, pathogens become increasingly resistant to the drugs and hence it is a severe threat to the both aquatic and terrestrial organisms. www.nature.com/scientificreports/ These toxic, non-biodegradable pollutants are difficult to degrade/mineralize under natural conditions. Since last two decades many physical, chemical and biological techniques which have been developed to degrade/ remove these contaminants from wastewater have disadvantages like high cost, longer time of degradation, and other pollutant parameters. Among various advanced oxidation processes (AOPs), photocatalytic and photofenton-like catalytic processes have attracted remarkable attention for the decomposition of organic effluents and antibiotics in efficient ways, the processes being both economically feasible and energy efficient. The photocatalytic process involves redox reactions initiated by electron(e − )-hole (h + ) pairs (generated by catalyst under light irradiation) 16 leading to the formation of active species. These active species are responsible for degradation of pollutants 17 . The photo-fenton-like catalytic process is a conventional fenton process in presence of light irradiation. In fenton process, •OH radicals can be generated by reaction between Fe-based catalysts (Fe 3 O 4 , BiFeO 3 ) and fenton reagent (eg: H 2 O 2 ). The additional light irradiation on fenton-process leads to generation of more •OH radicals 18 . The synergistic effect between the photocatalysis and fenton reactions enhances the photodegradation of effluents [19][20][21] . Fe-based visible light active photocatalysts could be promising candidates for photo-fenton-like catalytic processes, which can absorb 45-50% of sunlight from entire solar spectrum, whereas ultraviolet (UV) light active photocatalysts absorbs only 3-5% of sunlight 22 . Hence it is necessary to develop visible light-driven photo-fenton-like catalysts for wastewater treatment applications.
Perovskite BiFeO 3 (BFO) is one of the well-known multifunctional material, which has a wide range of applications due to its promising magnetic, electrical and optical properties. In recent times BFO and its composites have been widely explored as photocatalyst for water splitting and wastewater treatment as well [23][24][25][26][27][28] . The bandgap of BFO (2.1-2.6 eV) falls under visible range of solar spectrum and has a theoretical photo conversion efficiency about 7% 29,30 . If the bandgap can be reduced further, it is expected that the efficiency can be enhanced improving the photodegradation performance of catalyst. Another strategy to improve the catalytic activity is creation of substantial oxygen vacancies in perovskite structures, acting as active sites for catalytic activity [31][32][33] . In this regard, materials with a combination of low bandgap and oxygen deficiency, such as, oxygen deficient perovskite structured/brownmillerites can be explored 34,35 . Brownmillerite oxides such as Ca 2 Fe 2 O 5 , Ca 2 Mn 2 O 5 and Sr 2 Fe 2 O 5 show better catalytic activity over perovskite compounds due to substantial oxygen vacancies in their structure [36][37][38][39] . KBiFe 2 O 5 (KBFO) is one such recent brownmillerite compound which has smaller bandgap than BFO and showed promising photocatalytic activity to degrade organic effluents 40 . Nitrogen doping in KBFO can further enhance the photo-fenton activity due to presence of Fe-N x active sites and reduced bandgap over bare KBFO.
Recent studies have revealed that addition of chelating agent L-Cysteine to Fe-based catalysts Fe 3 O 4 and BiFeO 3 enhances the catalytic activity 41,42 . L-Cysteine is a sulfur-containing amino acid with three functional groups (-SH, -NH 2 , and -COOH) 42 . Reaction of L-Cysteine with O 2 is reported to generate H 2 O 2 , which acts as the fenton reagent. Hence, N-KBFO/H 2 O 2 /L-Cysteine system could well be proposed as a promising candidate for efficient photo-fenton activity and decomposition of organic effluents and antibiotics 41,42 .
In this work N-KBFO with various N-doping concentrations has been synthesized by sol-gel method. The structural, morphology, optical properties of as prepared samples were analyzed and detailed photo-fenton activity of N-KBFO in the presence of L-Cysteine and H 2 O 2 were investigated by degrading organic effluents Methylene blue (MB), Bisphenol-A (BPA) and antibiotics Norfloxacin (NOX) and Doxycycline (DOX) under visible light. The active species responsible for degradation of organic effluents are investigated using active species trapping experiment. The photoactivity of this N-KBFO and KBFO was also demonstrated using photoelectrochemical studies.

Experimental section
Preparation of N-KBFO. N-KBFO compound was prepared by conventional sol-gel technique. DFT calculations. In order to estimate the theoretical bandgap of KBFO and N-doped KBFO Density functional theory (DFT) calculations were conducted. DFT calculations are performed using the ultrasoft pseudopotential (USPP) method in the Quantum ESPRESSO package 43 . The exchange correlation energy is approximated using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional 44 . We have used the plane wave energy cutoff of 130 Ry and a 4 × 4 × 4 Monkhorst-Pack grid 45 . Self-consistency in calculations is achieved until the total energies have converged to 10 -6 eV/cell, and the structures have been relaxed until the Hellman-Feymann forces relaxed to less than 10 -2 eV/Å. The electronic structure is calculated by sampling the Brillouin zone with a set of high symmetry k-points 46 .  where E CB and E VB are the conduction and valence band edge positions, χ and E C are the absolute electronegativity of compound and energy of free electron on hydrogen scale (4.5 eV) respectively. E g is the corresponding bandgap energy.

Results and discussions
Theoretical bandgaps of KBFO and N-KBFO were calculated using density functional theory (DFT) calculations (Fig. 5a&b). DFT calculations were performed by sampling the Brillouin zone with a set of high symmetry k-points. The effect of Nitrogen doping in KBFO was analysed computationally and the bandgap of KBFO was found to be 1.59 eV. Upon replacement of few O atoms with N atoms in a unit cell of KBFO, the bandgap reduced to 1.18 eV, strongly supporting the experimental trend.
The catalytic activity of N doped KBFO samples were studied by degrading organic effluents MB and BPA as well as persistent antibiotics NOX and DOX. Photocatalytic degradation profile of MB by KBFO with various N-doping concentrations is shown in Fig. 6a. 20N-KBFO samples show better degradation efficiency (~ 84.5%), much higher than 41.6% for pure KBFO [ Fig. 6(b)]. An increase in the photodegradation efficiency www.nature.com/scientificreports/ upon increasing N concentration in KBFO may be attributed to the narrow bandgap and efficient charge separation in N doped samples due to the presence of Fe-N active sites 34 . N-doping in KBFO shifts the absorption edge to enable it to absorb more sunlight as compared to bare KBFO. The modification of perovskite structures with transition metal-N active sites is desirable to enhance the charge transport features enabling higher catalytic activity towards remediation of wastewater. With an increase in N concentration over and above 20 mmol, the degradation efficiency starts decreasing and the results are consistent with optical absorption studies. Excess N incorporation induces defect levels in KBFO, which act as recombination centers, thus reducing the photodegradation efficiency 52 . Hence the optimum N incorporation was confined to 20 mmol. The photocatalytic process is mainly governed by electron-hole (e -_ h + ) pairs generated in the catalyst upon light illumination and are responsible for redox reactions which mineralize the effluents. The photocatalytic mechanism of 20N-KBFO can be further enhanced by adopting fenton reactions with the addition of H 2 O 2 in optimum quantity. Addition of H 2 O 2 to an aqueous system containing an organic effluent and ferrous (Fe 3+ /Fe 2+ ) ions lead to occurrence of complex redox reactions. The hydroxyl radicals and superoxide radicles generated in this process attach with the complex organic molecule and mineralize into nontoxic byproducts. The reversible redox reactions generate Fe 3+ /Fe 2+ ions and these reactions take place until effluents degrade completely. Recent studies have revealed that in addition to chelating agents like sulfur containing amino acid, L-cysteine improves the photo-fenton activity which allows generation of •OH active species by reacting with O 2 and thus improve the catalytic performance. The optimization of dosage of fenton reagents (H 2 O 2 and L-Cysteine) in photocatalysis enhances the performance as well as economic feasibility. In this work, H 2 O 2 and L-cysteine dosage was optimized and found to be 1.5 mg/L and 10 mg/ml respectively. Upon addition of H 2 O 2 the degradation efficiency of 20N-KBFO improved from 84.5 to 92.7% while with L-cysteine it improved to 94.7% [Fig. 6c]. The photo-fenton performance was also tested through different combinations of fenton reagents as shown in Fig. 6(d). MB almost degraded completely (99.5% with a rate constant about 0.042 min -1 ) post addition of both H 2 O 2 and L-cysteine, which is only 9% for H 2 O 2 + L-cysteine without any catalyst. These investigations imply that 20N-KBFO + H 2 O 2 + L-cysteine combination is the best system for photo-fenton reaction for degrading MB. The degradation profile and first order reaction kinetics plot are shown in Fig. 6 (e & f).
In order to examine the active species involved in photo-fenton reaction, active species trapping experiments were conducted using various scavengers such as AgNO 3 , ethylenediaminetetraacetic acid (EDTA), isopropyl alcohol (IPA) and benzoquinone (BQ) and shown in Fig. 7. Sample 20N-KBFO + H 2 O 2 + L-cysteine showed a photodegradation efficiency of about 99.5% without any scavenger. When AgNO 3 (1 mmol) and IPA (1 mmol) were added to dye-catalyst suspension as eand •OH radical trapping agents, the photodegradation efficiency  ) radicals in photo-fenton mechanism is negligible. The photo-fenton mechanism is thus mainly governed by eand •OH radicals. The major contribution of •OH radicals in this mechanism is due to addition of H 2 O 2 and L-cysteine. A plausible degradation mechanism is illustrated in Fig. 8. The recyclability and stability of 20N-KBFO sample was investigated for three cycles. In all the three cycles, photodegradation performance of 20N-KBFO is negligible (Fig. 9a). The XRD pattern (Fig. 9) of recycled 20N-KBFO reveal that there are no structural transformations and secondary phases post three cycles of usage, stressing on the fact that the as prepared samples are reusable and stable for photocatalytic degradation of organic effluents.
20N-KBFO + H 2 O 2 + L-cysteine combination was further used to degrade the organic synthetic compound Bisphenol-A (BPA) under visible light. After exposing BPA-catalyst suspension in visible light for 120 min, BPA could be degraded upto 83% of its initial concentration with a rate constant of k = 0.016 min -1 whereas BPA alone degraded upto 2% only. The degradation profile and C/C 0 plot ratio plots are shown in Fig. 10a,b. 20N-KBFO + H 2 O 2 + L-cysteine was also used for degrading antibacterial effluents such as NOX and DOX under visible light. NOX and DOX degraded by 72% (k = 0.011 min -1 ) and 95% (0.026 min -1 ) of its initial concentration. The degradation profile and C/C 0 ratio plots are shown in Fig. 11a,b. These photo-fenton reaction studies with N-doped KBFO is a potential candidate for treating various effluents under sunlight.
The photoactivity of KBFO and 20N-KBFO were investigated and compared by photoelectrochemical (PEC) studies in 1 M Na 2 SO 4 aqueous electrolyte solution. Linear sweep voltammetry (LSV), Chronoamperometry (CA) and electrochemical impedance spectroscopic (EIS) studies were carried out under dark and light illumination.

Conclusion
Nitrogen doped KBiFe 2 O 5 was successfully synthesised using melamine (C 3 H 6 N 6 ) as the N source. Systematic investigations on structural, morphology and optical properties of as prepared samples were carried out. Optimum nitrogen incorporation in KBFO was analysed by degrading MB and 20 mmol of N doped KBFO was found to be the best sample for photo-fenton activity. Combination of H 2 O 2 + L-cystyein was used as fenton reagent and the photo-fenton activity in presence of 20N-KBFO + H 2 O 2 + L-cysteine showed rapid improvement in photodegradation efficiency by generating more active species like •OH (as confirmed from active species trapping experiments). Reusability and stability studies were performed upto three cycles and the samples show stable catalytic performance without any structural change. The performance of 20N-KBFO, LSV, CA and EIS studies revealed an enhanced photoresponse in 20N-KBFO over pure KBFO. Lower bandgap, high photodegradation efficiency, stability and satisfactory photoresponse exhibited by N-doped KBiFe 2 O 5 make it one of the best brownmillerite compound for energy and environmental applications.  www.nature.com/scientificreports/